Transcript Slide 1
16.451 Lecture 15: Beta Decay Neutron beta decay: 23/10/2003 n p e e light particles or “leptons”, produced in association. Neutrino presence is crucial to explain the shape of the electron energy spectrum: (otherwise, the electrons would be monoenergetic – 2 body final state! ) • “Neutrino” or “little neutral one” postulated in 1931 by Pauli (q = 0, m = 0, s = ½ ) • Only associated with the weak interaction – very difficult to detect • First detected by Reines & Cowan, 1959 Nobel prize 1995 1 Related processes: 2 1) p n e e “+ decay” in a nucleus, where energetically favourable, eg 25Al25Mg decay 2) p e n e “Electron capture” or EC decay in a nucleus; inner shell atomic electron is captured. 3) e p n e “Antineutrino capture”, used by Reines & Cowan to detect the antineutrino. ... and many more!!! Notice: the electron and anti-neutrino appear together; the positron and neutrino appear together.... This suggests a new conserved quantity called “lepton number”, Le : e have Le 1 ; e e have Le 1 e Empirical conservation law: Le = constant e and e are distinct!! More on lepton number: 3 There are actually three “generations” of leptons that we know about (in order of increasing mass: e, , ) and each one has its own distinct associated neutrino type with a separately conserved lepton number.... Le L L Example: muon decay: two distinct neutrinos are emitted, as proved by the spectrum shape PDG listing: e e Back to the electron-type neutrinos: e p n e 4 “Antineutrino capture” reaction used by Reines & Cowan to detect the antineutrino. Nobel-prize winning experiment: http://www.nobel.se/physics/laureates/1995/illpres/neutrino.html (Physical Review 117, p. 159, 1960) 5 Reines & Cowan Experiment (1953): intense beam from a nuclear reactor e p n e protons in large water tanks detect via gamma rays from annihilation with e- slow down by scattering in the water; detect by capture in dissolved Cd salt • Very low rate experiment: > 1013 incident antineutrinos/sec but only 3 events/hr! 5 months of data taking! • No computer data acquisition! For each event, an automatically-triggered camera system took a photograph of analog oscilloscope traces! • “Delayed coincidence” detection of both neutron and positron suppressed background • Auxiliary measurements to determine detection efficiencies, etc. • Absolute cross section measured was 1 x 10-43 cm2 (10-19 b), in agreement with theory!! Schematic of the experiment: 6 e p n e delayed: neutrons have to slow down... positron annihilation (instantaneous): e e 2 (511keV) 2 water tanks, sandwiched between 3 scintillation counters to detect ’s 7 e p n e vertical height ~ 2 m; surrounded in Pb shielding to reduce background... No computers! electronic coincidence pulses for e+ and n-capture ’s.... 8 events captured on analog oscilloscope displays.... Raw data: oscilloscope photographs! scintillation light from e+ annihilation first, neutron capture later ( 3 – 10 s) 9 Data: coincidence event rate as a function of time delay 10 distribution indicates slowing-down time of neutrons in water – 1/v Cd capture cross section is large at low energy! 1.2 0.7 0.4 1043 cm2 Bottom line: first direct demonstration of the existence of antineutrinos! Parity Violation in Beta Decay 11 Famous experiment carried out by C.S. Wu (1957) at the suggestion of Lee & Yang (1956, Nobel Prize 1957) demonstrated that the weak interaction violates parity 60 60 Co 27 28 Ni e e Key observation: when cobalt nuclei were polarized in a magnetic field at low temperature, electrons were emitted preferentially in a direction opposite to the nuclear spin... J pR 60 27 Co “before” ( J 5 ) 60 28 Ni pe e e ( J 4 ) “after” Data from Krane, Chapter 9: 12 A pseudoscalar observable: J pe 13 electron emission angle: Under a parity transformation: Angular momentum: Linear momentum: ~ J pe r r dr d r J ~ r r ~ J dt dt dr d r p ~ ~ p dt dt Jp Jp Observer using a parity-reversed coordinate system deduces the opposite correlation of e- and J... but this is “crazy”.... ???? Consider what a parity transformation does to a coordinate system: r r 14 iˆ ˆj kˆ “Normal” RIGHT-handed Cartesian system: z x’ = -x y’ = -y y x z’ = -z Reverse of coordinate axes: x’ = -x, etc. the system is LEFT-handed: iˆ' ˆj ' kˆ' Principle of parity conservation: 15 Laws of physics should be independent of coordinate system! In particular, a right-handed and left-handed choice of Cartesian coordinates should be completely arbitrary. (We should get the same answer both ways.) (True for gravity, strong, and electromagnetic interactions) This is not true for the weak interaction: Jp has the opposite sign in LH and RH systems by demonstrating a preferred correlation Jp , beta-decay “prefers” a LH coordinate system symmetry is broken! In fact, the electron and antineutrino themselves show a similar correlation: s. p define “helicity” h: for a particle with spin s, h , 1 h 1 and momentum p sp Electrons emitted in -decay have h = -v/c “left handed” (positrons “ “ h = +v/c “right handed”) Neutrinos have h = -1 (LH) and antineutrinos have h = +1 (RH) -- this is the only perceptible difference between them!!!!! Parity Nonconservation and the Standard Model: 16 Weak force carriers, W+, Z°, W- have spin – 1 (bosons) and are left-handed, i.e. they have h = -1 always (spin in the direction of motion, never opposite) If this is the case, then parity violation in the weak interaction is a “built-in” feature. But nobody knows why.... Extensive searches for physics “Beyond the Standard Model” probe the existence of a symmetric set of right-handed force carriers. None detected yet, but if they exist, they are required to be extremely heavy!